1. Field of the Invention
This invention relates to systems for monitoring vital signs, and particularly blood pressure, during hemodialysis.
2. Description of the Related Art
Patents in late-stage renal failure typically require hemodialysis for survival. During a hemodialysis treatment, blood is extracted from a patient's veins to remove excess water and waste products, such as potassium and uric acid, with a process that combines diffusive clearance across a membrane (dialysis) and convective clearance (ultrafiltration). Rapid extraction of fluid can cause the patient's blood pressure to quickly decrease due to the lack of volume in the vessels. This can also increase or reduce the patient's heart rate, increase their body temperature, and induce nausea and severe fatigue. In some cases these side effects can be life-threatening. Frequent hypotensive episodes, for example, have been linked to increased mortality in the dialysis population.
A method known as pulse transit time (PTT) can continuously measure a patient's blood pressure with only intermittent calibration with a cuff-based system. PTT, defined as the transit time for a pressure pulse launched by a heartbeat in a patient's arterial system, has been shown in a number of studies to correlate to both systolic (SYS) and diastolic (DIA) blood pressure. In these studies, PTT is typically measured with a conventional vital signs monitor that includes separate modules to determine both an electrocardiogram (ECG) and pulse oximetry value (SpO2). During a typical PTT measurement, multiple electrodes attach to a patient's chest to determine a time-dependent ECG waveform characterized by a sharp spike called a ‘QRS complex’. The QRS complex indicates an initial depolarization of ventricles within the heart and, informally, marks the beginning of a heartbeat and a pressure pulse that follows. Pulse oximetry is typically measured with a bandage or clothespin-shaped sensor that attaches to a patient's finger, and typically includes optical systems operating in both the red and infrared spectral regions. A photodetector measures radiation emitted from the optical systems that transmits through the patient's finger. Other body sites, e.g., the ear, forehead, and nose, can also be used in place of the finger. During a measurement, a microprocessor analyses both red and infrared radiation measured by the photodetector to determine the patient's blood oxygen saturation level and a time-dependent optical waveform called a photoplethysmograph (PPG). Time-dependent features of the optical waveform indicate both pulse rate and a volumetric absorbance change in an underlying artery (e.g., in the finger) caused by the propagating pressure pulse.
Typical PTT measurements determine the time separating a maximum point on the QRS complex (indicating the peak of ventricular depolarization) and a foot of the optical waveform (indicating the beginning the pressure pulse). PTT depends primarily on arterial compliance, the propagation distance of the pressure pulse (which is closely approximated by the patient's arm length), and blood pressure. To account for patient-dependent properties, such as arterial compliance, PTT-based measurements of blood pressure are typically ‘calibrated’ using a conventional blood pressure cuff. Typically during the calibration process the blood pressure cuff is applied to the patient and used to make one or more blood pressure measurements. Going forward, the calibration blood pressure measurements are used, along with a change in PTT, to determine the patient's blood pressure and blood pressure variability. PTT typically relates inversely to blood pressure, i.e., a decrease in PTT indicates an increase in blood pressure.
A number of issued U.S. patents describe the relationship between PTT and blood pressure. For example, U.S. Pat. Nos. 5,316,008; 5,857,975; 5,865,755; and 5,649,543 each describe an apparatus that includes conventional sensors that measure an ECG and optical waveform, which are then processed to determine PTT.